It is common practice to extrude plastic in the form of a large tube, the tube being flattened and slit longitudinally thereof, and the cut edges being sealed together to form two smaller tubes. The conventional apparatus for accomplishing this slitting and sealing comprises simply a cutting blade that is heated sufficiently to melt the plastic as the plastic moves over the blade. The most popular blade for slitting and sealing has a vertically straight, or sometimes notched, cutting edge, and has grooves on each side thereof, the plastic riding in the grooves so that a bead is rolled along the edge of the material in an effort to make a secure seal.
Many of the difficulties with the prior art slit-sealers have been solved by the method and apparatus disclosed in the above identified co-pending application; however, with the high-density plastics, the problem has not been solved. High-density plastics present additional problems in that the material is more difficult to seal. Furthermore, while high-density plastic is generally much stronger than the low-density material, once a tear is started in high-density plastic, the material continues to tear very easily. The result is that even pin holes in the seams of high-density material cannot be tolerated. Thus, with the high-density plastics, it is more difficult to seal, and the standards for the seals must be maintained at a higher level.
The prior art slit-sealing of high-density material has usually been accomplished by the same apparatus as the slit-sealing of the low-density material. The changes that have been made have usually included a higher temperature on the heated knife and/or slower film speeds to achieve a greater dwell time in an effort to get more heat into the film as it drags over the knife. Since the knife is generally heated by passing an electric current therethrough, the knife blade itself being made of nickel-chromium-steel, the higher temperature means a greater current, and the greater current is undesirable in the manufacturing environment. Also, with the higher temperature and slower speeds, the material may burn, at least in spots, causing weakened portions of the seam which can cause the material to split readily.